Heat-developable silver halide photothermographic imaging materials, often referred to as "dry silver" compositions because no liquid development is necessary to produce the final image, are known and disclosed in, e.g., U.S. Pat. Nos. 3,152,904, 3,457,075, 3,839,049, 3,985,565, 4,022,617 and 4,460,681, and in "Thermally Processed Silver Systems" by D. Morgan and B. Shely, Imaging Processes and Materials, Neblette's Eighth Edition, Edited by Sturge et al., (1969). Such materials generally comprise: a light-insensitive, reducible silver source; a light-sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light-sensitive material is generally photographic silver halide which must be in catalytic proximity to the light-insensitive silver source. "Catalytic proximity" is defined as an intimate physical association of the two materials such that when silver specks or nuclei are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has long been understood that silver is a catalyst for the reduction of silver ions and the silver-generating, light-sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., as disclosed in U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., as disclosed in U.S. Pat. No. 3,839,049) and any other method which intimately associates the silver halide and the silver source. Exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image is generally not visible by ordinary means and the exposed material must be further processed in order to produce a visual image. Although stable at ambient temperatures, when heated after imagewise exposure to higher temperatures, silver is produced in the exposed regions of the medium through a redox reaction between the reducible silver source (acting as an oxidising agent) and the reducing agent. This redox reaction is accelerated by the catalytic action of the exposure generated silver atoms. The silver contrasts with the unexposed areas to form the image. Alternatively, the reducing agent may be such that it generates a colour on oxidation, either by becoming coloured itself, or by releasing a dye during the process of oxidation. The resulting colour image may optionally be diffused thermally to a separate receptor layer.
Photothermographic materials, like other light-sensitive systems, tend to suffer from fog. This spurious image density appears in the non-developmentally sensitized areas of the material and is often reported in sensitometric results as Dmin. This problem is also related to certain stability factors in the photosensitive material where fog increases upon storage. It is therefore customary to include an effective antifoggant in these materials.
In the past, the most effective antifoggant has been mercuric ion. The use of mercury compounds as antifoggants in photothermographic materials is disclosed in, e.g., U.S. Pat. No. 3,589,903. However, mercury compounds are environmentally undesirable and due to increasing pressure to remove even trace amounts of possible pollutants from commercial articles there is a demand to find equally effective but less hazardous antifoggants. Various compounds have been suggested for use as antifoggants in place of mercury compounds in photothermographic materials.
U.S. Pat. No. 4,546,075 discloses the use, in photothermographic media comprising an inorganic silver salt, a photocatalyst and a reducing agent, of compounds of the following general formulae as antifoggants in place of mercury compounds: ##STR2## in which; n has integral values of from 1 to 4,
Q represents S, O or NR.sup.2, PA1 R.sup.1 represents hydrogen or an alkyl, aryl, aralkyl, acyl, carbamoyl, alkylsulphonyl or arylsulphonyl group or a heterocyclic ring or fused ring nucleus, PA1 each R.sup.2 independently represents an alkyl, aryl or acyl group, and PA1 X represents a halogen atom. PA1 each X represents a halogen atom, preferably bromine, and PA1 R.sup.1 and R.sup.2 independently represent an acyl, oxycarbonyl, oxysulphonyl, alkylsulphonyl, arylsulphonyl, aralkylsulphonyl, carboxy, sulpho or sulphamoyl group, each of which may optionally be substituted. PA1 R.sup.1 represents hydrogen or an alkyl, aryl, aralkyl or alkenyl group or a heterocyclic ring or fused ring nucleus, each of which may be substituted. PA1 X.sup.3 represents a halogen atom, such as bromine or chlorine, preferably bromine, or an electron-withdrawing substituent, e.g., acyl, oxycarbonyl, oxysulphonyl etc., and PA1 Z represents the necessary atoms to complete an optionally substituted heterocyclic ring or fused ring nucleus. PA1 R.sup.3 is hydrogen, halogen or a cyano group, and PA1 R.sup.4 is hydrogen or an alkyl group, generally comprising up to 10 carbon atoms, preferably not more than 5 carbon atoms. PA1 each R.sup.5 represents a substituent selected from those defined for groups represented by R, e.g., alkyl groups, alkoxy groups, aryl groups, nitro, cyano, and the like. Substituents on adjacent positions may form fused ring groups so that formula (i) above would in fact be inclusive of formulae (ii) and (iv). PA1 R.sup.7 and R.sup.8 represent alkyl groups of up to 5 carbon atoms, e.g., methyl, ethyl, t-butyl etc.
U.S. Pat. No. 4,546,075 also discloses the use of tribromoacetophenone as a comparative antifoggant when assessing the efficiency of the aforedescribed tetrazole, benzothiazole, benzoxazole and benzomidazole compounds. The results presented show that tribromoacetophenone has a negligible effect on the level of fog generated in the exemplified photothermographic system. For example, referring to Example 1, tribromoacetophenone achieves only minimal reduction on the level of fog observed in the control medium containing no antifoggant (a decrease in Dmin of from 0.69 to 0.55) when compared with the level of fog reduction achieved by the various tetrazole compounds etc., of the invention (Dmin of between 0.08 to 0.22 variously).
Japanese Patent Publication No. 59-57234 discloses, as antifoggants in place of mercury compounds, the use of compounds of the general formula: EQU R.sup.1 --CX.sub.2 --R.sup.2
in which;
U.S. Pat. No. 4,452,885 discloses, as antifoggants in place of mercury compounds, the use of compounds of the general formula: ##STR3## in which; X represents a halogen atom, and
European Patent Publication No. 223606 discloses, as antifoggants in place of mercury compounds, the use of compounds of the general formula: ##STR4## in which; X.sup.1 and X.sup.2 independently represent halogen atoms, preferably bromine,
Japanese Patent Publication No. 61-129642 discloses the use of halogenated compounds (including phenyl-(.alpha.,.alpha.-dibromobenzyl)-ketone to reduce fog in color-forming photothermographic emulsions.
U.S. Pat. No. 3,767,399, British Patent No. 1398265 and European Patent Publication No. 26859 disclose colour imaging systems in which organohalogen compounds, including tribromomethyl ketone compounds, are photolysed on exposure to light to produce a halogen radical which oxidises a colour-forming compound, e.g., an aldol naphthylamine, a leuco dye etc., to produce a coloured image.
European Patent Publication No. 061898 discloses the use of tribromomethyl ketone compounds as photoinitiators for a thermally developed imaging medium comprising a leuco dye, a nitrite ion and a sensitising dye.
Belgian Patent No. 876734 discloses the use of tribromomethyl ketone compounds to reduce the fog level in conventional, `wet-processed` silver halide based imaging media, as well as claiming a speed enhancement.
Japanese Patent No. 61-93451 discloses aqueous silver halide/silver benzotriazole based imaging media incorporating water-soluble sensitising dyes and other conventional photographic additives. The imaged material is not thermally processed, but `fixed` by contact with another coating to which the dye image is transfered. Certain tribromomethyl ketone compounds are disclosed as antifoggants for use therein.
French Patent Nos. 2483092 and 2483637 and British Patent Nos. 2076552 and 2076984 disclose silver iodide based photothermographic media of the post-activation type, i.e., requiring thermal activation prior to imaging, incorporating as antifoggants an oxidising agent for free silver and a photo-reactive organohalogen oxidising agent comprising a halogenated organic compound having one or more bromine-carbon or iodine-carbon linkages. The preferred organohalogen oxidising agent is o-tetrabromoxylene, although a number of tribromomethyl ketone compounds are exemplified. The free silver oxidising agent, usually mercuric ion, although palladium and cobalt are also exemplified, is the primary antifoggant with the organohalogen oxidising agent functioning in a secondary role to regenerate the reduced free silver oxidising agent.
Tribromomethyl ketone compounds have now been found to be effective antifoggants in photothermographic materials of the type disclosed in U.S. Pat. No. 5,028,523, which contain, in addition to the usual photothermographic chemistry, a hydrobromic acid salt of a nitrogen-containing heterocyclic ring or fused ring nucleus associated with a pair of bromine atoms, as a speed enhancing agent/antifoggant.